This invention relates to a process for reacting isocyanate compounds containing isocyanate moieties or active hydrogen moities in the presence of quaternary ammonium zwitterions which initiate the reaction.
Reactions of isocyanates with active hydrogen compounds such as polyols to form polyurethanes are well known as described in Polyurethanes: Chemistry and Technology I., Saunders and Frisch, Interscience, pp. 66-128 (1962). Such reactions are commonly catalyzed by tertiary amines, organometallic compounds, phosphines, triazines and mixtures thereof. Such polyurethanes are usually produced of elastomers, coatings, adhesives, fibers, films and most often as flexible and rigid foams.
In the production of foams, the cellular polyurethane is made by reacting an isocyanate with polyol, generally a polyfunctional hydroxyl terminated polyether or polyester, in a formulation also containing a blowing agent and a surfactant in addition to the reactants and the catalyst. By proper choice of amounts and types of the isocyanate and polyol, foams can be made with properties ranging from flexible to semirigid to rigid. As is well known, the polyurethane foams, particularly the rigid foams, are excellent thermal insulators in a wide variety of applications, especially in the refrigerant and construction industries.
Unfortunately, the flammability and smoke evolutionary characteristics of the polyurethane foams have significantly limited the use of such foams in many applications. Conventionally, the tendency of polyurethane foam to burn is reduced by adding halogenated organic materials or combinations thereof with phosphorus compounds to the polyurethane formulation. More recently, polyurethanes having improved flame and heat resistance (hereinafter called "thermal stability") have been made by introducing more isocyanurate groups into the polyurethane. In addition to high thermal stability, polyurethanes containing such isocyanurate groups also exhibit hydrolytic and dimensional stability.
Such foams are normally produced by employing so-called trimerization catalysts in combination with conventional urethane catalysts in the urethane formulation. Exemplary trimerization catalysts include the amine catalysts such as 2,4,6-(N,N-dimethylaminomethyl)phenol; hexahydrotriazines; metal alkoxides; metal carboxylates; metal oxides; organometallics; metal chelates and combinations thereof. Unfortunately, relatively large amounts of catalysts are required to produce the trimerization needed to form isocyanurate groups, particularly in the preparation of isocyanurate foams. The presence of larger amounts of catalysts, particularly the amino or metal catalysts, often causes degradation when the resultant foam is subjected to elevated temperatures. Some catalysts, particularly the alkali metal carboxylates, yield foams which are so friable that they exhibit little, if any, resistance to impact and abrasion. More recently, as taught in U.S. Pat. No. 4,111,914, isocyanurate foams exhibiting reduced friability have been prepared from stable isocyanurate oligomers formed in the presence of sulfonium zwitterions. While such isocyanurate foams exhibit many desirable properties as compared to conventional isocyanurate foams, further improvements in reaction rates, processability, impact and abrasion resistance are desired.
In the preparation of flexible polyurethane foams wherein foaming is caused in part by carbon dioxide released from the reaction of water with isocyanate groups, a dual catalyst is required in order to obtain a foam having acceptable physical properties and/or to facilitate processing. This dual catalyst commonly consists of a tertiary amine and an organometallic compound. Due to the instability of the organometallic compound, usually a tin alkanoate such as tin octoate, these dual catalysts have to be carefully metered in separate streams in commercial foam machines. Also, the amine compounds used as one of the components of the dual catalyst are often toxic and cause undesirable odor problems.
In view of the aforementioned difficulties with the conventional catalysts employed and the reactions of isocyanates, it is highly desirable to provide a process for preparing polyurethane foams, particularly those containing substantial isocyanurate moieties, wherein such foams exhibit good thermal resistance as well as impact and abrasion resistance. In addition, it is desirable that improvements in reaction rates and processability be achieved by such process.